Cook noodles according to package directions until just tender, slightly undercooking them since they'll finish cooking with the beef later. Drain and rinse with cold water to stop the cooking process.

Whisk together soy sauce, brown sugar, rice vinegar, garlic, ginger, and sesame oil in a small bowl until well combined.

Toss sliced flank steak with cornstarch until evenly coated. Let it sit for 2-3 minutes while your pan heats up.

Heat 2 tablespoons oil in a large skillet or wok over medium-high heat until it shimmers. Add beef in a single layer, working in batches if necessary. Let it sear undisturbed for 2 minutes until golden brown, then stir-fry for another 2-3 minutes.

Push beef to one side, add remaining oil and sliced onion. Cook for 2 minutes until softened but still crisp. Pour in the prepared sauce and add cooked noodles, tossing everything together for 2-3 minutes until the sauce coats everything beautifully and the noodles are heated through.